Hi Guys, long time lurker first time poster.
I have a 1986 Nissan 720 4x4 King Cab 5 speed manual that I purchased about 6 months ago with 115k miles. After buying it, I noticed there was a weird vibration when the car with in 4th or 5th speed and only at 3,000rpm for some reason. The clutch started slipping last week, so I took it to a local mechanic I'd never been to, but had good reviews. I mentioned to him the vibration issue, and he recommended replacing the U joints, which was something I had been planning on doing anyway, so he swapped 4 of the 6 U joints on the drive shaft.
I received the car back yesterday and when driving on the freeway, the vibration is now way more pronounced from 2,000 rpm and up in 3rd, 4th, and 5th gears. At 1st and 2nd gears, it doesn't vibrate at any rpm. Any ideas on what could be wrong?